Conventionally, apparatuses which apply radiation to targets, detect the intensity distributions of radiation transmitted through the targets, and obtain radiographic images of the targets have been widely and generally used in the fields of industrial nondestructive testing and medical diagnosis. As a general method for such radiography, a film/screen method using radiation is available. This is the method of performing radiography by using a combination of a photosensitive film and a phosphor having sensitivity to radiation.
In this method, rare-earth phosphor sheets which emit light upon application of radiation are held in tight contact with the two surfaces of a photosensitive film, and the phosphor sheets convert radiation transmitted through a subject to be radiographed into visible light. The latent image formed on the photosensitive film by making it capture this visible light is then developed, thereby visualizing a radiographic image.
With the recent advances in digital technology, the scheme of playing back the visible image obtained by processing the electrical signal obtained by converting a radiographic image on a CRT or the like has become widespread.
This method temporarily stores a transmission image of radiation as a latent image in a phosphor. There has been proposed a radiographic image recording/playback system which photoelectrically reads out the latent image as a visible image by applying exciting light such as a laser beam to the phosphor. In addition, with the recent advances in semiconductor process technology, an apparatus for capturing a radiographic image in the same manner by using a semiconductor sensor has been developed.
These systems have very wide dynamic ranges as compared with a conventional radiographic system using a photosensitive film, and can obtain a radiographic image which is robust against the influences of variations in the amount of radiation exposure. At the same time, unlike the conventional photosensitive film scheme, this method need not perform any chemical treatment and can instantly obtain an output image.
FIG. 13 is a view showing the arrangement of a radiographic system using the above semiconductor sensor. A radiographic apparatus 2 mounted on a radiographic stand 1 includes a radiation sensor 3 as a semiconductor sensor having a detection surface on which a plurality of photoelectric conversion elements are two-dimensionally arranged.
The phosphor placed on the radiation sensor 3 converts the radiation applied from a radiation generator 4 to a subject S into visible light, and the radiation sensor 3 images the light. A control unit 5 performs digital image processing for the electrical signal output from the radiation sensor 3. A monitor 6 then displays a radiographic image of the subject S on the basis of this processed image signal.
This radiographic system allows the operator to instantly observe an image. The detection panel of such a radiographic system is mounted on a holder specialized for a radiographic form such as radiography in a standing position or a resting position, and is selectively used as needed. This system is stationarily installed in a radiation room. Recently, a portable detection unit has been developed, and is used when it is necessary to radiograph a subject in an arbitrary radiographic posture.
Such a radiographic apparatus is electronic equipment, and hence includes many electronic parts indispensable to digitization, which inevitably pose the problem of heat generation, as compared with the conventional film/screen method. For this reason, it is necessary to efficiently dissipate heat from these electronic parts. Heat dissipation is very important to prevent a change in the characteristics of a radiation detector due to a rise in temperature inside the radiographic apparatus as well as to improve the normal operation and durability of the electronic parts which generate heat.
In addition, it is necessary to suppress a rise in the temperature of the outer jacket of the radiographic apparatus from the viewpoint of the safety of subjects in the field of medical equipment. As indicated by “Japanese Industrial Standard on Safety Standards for Electrical Medical Apparatus (JIS T0601-1)”, there is a restriction on the surface temperature of a portion with which a subject comes into contact.
Note that Japanese Patent Laid-Open No. 2000-37374 discloses an apparatus having a cooling mechanism of cooling the heat generated by a detection unit by drawing air through a vacuum port and circulating the air around the detection unit by driving a cooling fan. Japanese Patent Laid-Open No. 2005-370 discloses an apparatus having a cooling mechanism of switching heat dissipation paths to effectively dissipate heat in accordance with an installation form, for example, a standing position or a resting position.
There are demands for the application of a detection unit having a cooling mechanism like that described above to a radiographic apparatus for radiographic moving image. In the case of a radiographic moving image, since radiography is continuously performed, the amount of heat generated increases as compared with conventional radiography of a still image. That is, it is necessary to further improve the cooling performance.
There has also been a radiographic apparatus which independently uses a detection unit detachable from a holder as a cassette type unit instead of a stationary type unit. When this apparatus is to obtain radiographic moving image, in order to improve the cooling performance as described above, it is necessary to mount a new cooling mechanism in the detection unit. This reduces the merit of the compact, lightweight detection unit. In addition, in the case of a radiographic moving image, since the dose of X-rays increases, it is necessary to improve safety.